1. Field of the Invention
The present invention relates to an electric power calculation system for digital calculation of a current value or integrated value of electric power, in particular of reactive power, from received analog voltage and current.
2. Description of the Related Art
For an electric power supply line having a voltage V and a current I, with a phase angle "psgr" in between, an active power W is determined such that W=Vxc2x7Ixc2x7cos "psgr", and a reactive power Q, such that Q=Vxc2x7Ixc2x7sin "psgr". As Vxc2x7I sing "psgr"=xe2x88x92Vxc2x7Ixc2x7cos ("psgr"xe2x88x9290xc2x0), the calculation of reactive power is generally implemented by shifting an instantaneous voltage or an instantaneous current at 90xc2x0 in phase.
FIG. 1 shows a reactive power calculation system CSO performing a calculation of reactive power in a conventional electric power calculation system.
The calculation system CSO comprises a pair of AD (analog-to-digital) converters 201, 202, and a microcomputer 203.
The converter 201 samples a number of times equivalent to a necessary number of bits an analog signal V1 which is input from an input terminal T1 and directly proportional to an instantaneous voltage of a system under measurement, and converts it to a corresponding digital voltage date D1. The converter 202 samples an identical number of times an analog signal A1 which is input from an input terminal T2 and directly proportional to an instantaneous current of the system under measurement, and converts it to a corresponding digital current date D2.
The microcomputer 203 follows a program on an incorporated CPU (central processing unit), for executing phase shift and multiplication processes of the input data D1, D2 to calculate an instantaneous reactive power Q [Var], and for integrating this to calculate a reactive electric energy ∫Q [Varh].
In this system in which a phase shift process of voltage or current data is effected on software, it is required to repeat the phase shift process a number of times corresponding to the number of bits output from the converter 201 or 202, constituting a neck for the speed to be increased, in this concern, for a speed-up to be achieved by paralleled processes, there arises an increase in memory or register capacity, leading to a scaled-up configuration to be high of cost.
Moreover, the multiplication process on software needs many repeated processes to be executed within a fixed time interval in cycle, and parallel jobs to be burdened on the microcomputer 203, such as for a display process, causes the more scaled-up configuration.
Further, the use of converters 201, 202 gives rise to a progressive additional scale-up in configuration for AD conversion, when accompanying an intention for increasing the number of bits of digital data to have a raised number of significant figures.
The present invention has been made with such points in view. Accordingly, it is an object of the present invention to provide an electric power calculation system that can achieve, at a relatively low cost and high speed, the execution of a signal processing of analog voltage and current signals it has received for a digital calculation to determine a current or integrated value of electric power, in particular of a signal processing including a phase shift process for a digital calculation of reactive power.
It is desirable for the system to be configured compact with a possibly greater number of system elements implemented as hardware, and it is more desirable if an entire system can be implemented as an LSI.
An aspect of the present invention to achieve the object described is an electric power calculation system comprising a first system element for inputting a first analog signal representing a voltage alternating with a period and a second analog signal representing a current alternating with the period, and outputting a first digital signal representing an input value of the first analog signal and a second digital signal representing an input value of the second analog signal, the first system element including a phase-shifting circuit for phase-shifting one signal of the first and second analog signals and the first and second digital signals by holding a circuit status wholly representing a signal value of the one signal, and a second system element for executing a calculation process of the first and second digital signals output from the first system element to provide a third digital signal representing a reactive power associated with the voltage and the current.
According to the aspect of the invention, one of a pair of analog input signals representing a voltage and a current having an identical period or one of a pair of digital signals representing input values of the input signals is shifted in phase as necessary by a phase-shifting circuit in a system element, and correspondent digital signals are output from the system element to another system element, where they are processed for calculation 10 provide a digital signal representing a reactive power.
The phase-shifting circuit is adapted to have a circuit status that is wholly representative of a signal value of the signal to be shifted in phase, and to bold the circuit status for a necessary time interval for the signal to be phase-shifted, without the need for the phase shift process to be repeated in correspondence to the number of bits of the signal, even when this is a digital signal, thus allowing the speed to be the more increased.
Another aspect of the present invention to achieve the object is an electric power calculation system which coverts signals directly proportional to a voltage and a current of a system under measurement to digital values respectively using A-D converters to calculate a reactive power, wherein a rime delay is imparted to a digital value of the voltage or current to be phase-shifted by phase shift means using a semiconductor memory.
This aspect of the invention enable a redaction in analog portion of the system configuration, and provides a compact and low cost configuration, even when implemented as an LSI. It also minimizes and facilitates software processing.
Another aspect or the present invention to achieve the object is an electric power calculation system which coverts signals directly proportional to a voltage and a current of a system under measurement to digital values respectively using A-D converters to calculate a reactive power, wherein a time delay is imparted to a digital value of the voltage or current to be phase-shifted by phase shift means using shift registers.
This aspect of the invention also enable a reduction in analog portion of the system configuration, and provides a low cost configuration, even when implemented as an LSI. It permits an entire configuration to be implemented as hardware.
Another aspect of the present invention to achieve the object is an electric power calculation system which covens signals directly proportional to a voltage and a current of a system under measurement to digital values respectively using A-D converters to calculate a reactive power, wherein a phase shift is performed by a phase-shifting circuit formed by a capacitor, a resistor, and an operational amplifier, disposed before an A-D converter at a voltage side or current side.
This aspect of the invention also enable an electric power calculation system to be entirely configured as hardware.